To provide stable and reliable operation of a non-consumable electrode, it is essential that certain requirements to its cooling should be satisfied.
First of all, maximum temperature in an active insert should not exceed the temperature of evaporation of its material. This condition should be met on the most thermally loaded surface of the active insert in the portion thereof whereon an electrode drop region of an electric arc rests.
Then, the temperature of a yoke should not exceed the melting point of its material since under conditions of high mechanical stresses caused by large temperature gradients arising during operation of an electrode the yoke should possess sufficient mechanical rigidity.
Besides, a heat flow density on the surface of the yoke interior cooled by the liquid, i.e. the amount of heat passing through the unit of this surface, should not exceed a critical value above which the cooling system does not provide sufficient heat removal, and stable and reliable operation of an electrode is upset.
Known in the prior art is a non-consumable electrode, chiefly a cathode, intended for operation in different plasma-forming mediums (see the U.S. Pat. No. 3,198,932).
This non-consumable electrode comprises a water cooled electrode holder with a copper yoke fixed therein and an active insert made as a rod of cylindrical shape and fixed in said yoke, for example, by means of soldering or pressing.
Such a shape of the active insert provides stable and reliable operation of a non-consumable electrode at an arc current up to 300 amperes above which an electrode is overheated beyond maximum limiting temperatures and, as a result, destroyed.
The operating life of a non-consumable electrode can be increased by enlarging the area of the cooled surface of the yoke interior.
Also known in the prior art is a non-consumable electrode (see the Author's Certificate of the U.S.S.R. No. 420,222) comprising a water cooled electrically conductive housing with a yoke fixed therein and made of a highly heat conductive material, and an active insert of a cylindric shape fixed in said yoke, the water cooled surface of said yoke being formed as a parabola.
A parabolic-shaped water cooled surface of a yoke improves the intensity of cooling thereof, as a result of which the rate of burning-out of the active insert is reduced and the working length and operating life of a non-consumable electrode are increased.
However, due to enlargement of the area of the yoke portions nearest to the surface of heat removal means and found to be in conditions close to critical ones as regards the heat flow density of a cooling liquid, reliability of operation of a non-consumable electrode is decreased.
The aforementioned non-consumable electrodes exhibit low thermal resistance, and they can be employed for air plasma-arc cutting at a current only up to 300 amperes.
To characterize thermal resistance of a non-consumable electrode, the authors used the value of a limiting heat flow, i.e. a total amount of heat supplied to the electrode, which the non-consumable electrode can withstand with repeated energizings without destruction.
Thermal resistance of a non-consumable electrode can be increased by enlarging the surface of contact between the active insert and the cooled yoke.
Known in the art is a cathode of a DC plasma arc torch (see the U.S. Pat. No. 3,408,518), comprising an electrode holder, a yoke fixed therein, both being made of heat and electrically conductive materials and having a common interior whereinto a cooling liquid is supplied, and, an active insert rigidly fixed in the yoke and formed as a truncated cone whose major base faces the cooling interior and has a recess shaped as a peaked cone facing with its peak the portion whereon the electrode drop region of an electric arc rests.
When an active insert shaped as a truncated cone is used, besides the enlargement of the surface area of its contact with the cooled yoke, an additional positive effect is simultaneously achieved consisting in that the portions of the water cooled surface of the yoke, most stressed in terms of the values of taken thermal loads, are shielded by the active insert flaring in the direction toward this surface, the material of said active insert always possessing low heat conduction as compared to the material of the yoke.
However, the increase in the linear dimensions of the active insert in radial direction as a result of cone flaring reduces the efficiency of cooling of the central portion of the active insert. To compensate for this effect, formed in the surface of the active insert, facing the cooling interior, is a recess of a cone shape with its peak facing the portion whereon the electrode drop region of an electric arc rests.
The recess in the surface of the insert, facing the cooling interior, provides reduction of the normal distance from the surface whereon the electrode drop region of an electric arc rests and which actually takes all the heat flow from the arc to the yoke, and simultaneous enlargement of the surface of contact between the active insert and the yoke.
It is well known to those skilled in the art that, with such a shape of the recess due to its acute peak, a region of high temperature gradients and considerable mechanical stresses caused thereby appear around this peak, which inevitably hastens destruction of the insert under operation particularly when it operates under conditions of intermittent energizings during plasma arc cutting, welding and similar processes.